the seminiferous epithelium, the site of spermatogenesis, is populated by somatic Sertoli cells and developing spermatogenic cells. There is evidence that these cells interact extensively and it has been proposed that these interactions are essential for spermatogenesis. Recently, we described a new glycoprotein, Cyclic Protein-2 (CP-2), which is secreted by Sertoli cells in response to specific germ cells. Cyclic Protein-2 is the principal in vitro secretory product of Sertoli cells in Stage VI seminiferous tubules while it is secreted in minimal amounts at Stage XII. This protein is also secreted in vivo as it can be purified in microgram amounts from the seminiferous tubule fluid of a single male rat. In this application, we proposed the first thorough analysis of the composition, function, cellular distribution and biosynthesis of a protein which is secreted in a stage-dependent manner by Sertoli cells. We propose to characterize CP-2 both immunochemically and biochemically. We will identify, clone and sequence a cDNA for CP-2. Comparison of this sequence with that of proteins of known function may indicate the role of CP-2 in the seminiferous epithelium. Finally, we will examine the cellular distribution and biosynthesis of CP-2 at different stages of the epithelial cycle and we will measure the Sertoli cell's mRNA content for this protein at these different stages.